There has been conventionally known an electrophotographic image forming apparatus, such as a copier, a printer, and a facsimile device. The electrophotographic image forming apparatus forms an electrostatic latent image on a surface of a toner image bearing member, and then supplies a toner to the toner image bearing member by the developing device. This develops the electrostatic latent image so that a toner image is obtained. After that, the toner image formed on the toner image bearing member is transferred to a sheet such as paper, and then the toner image is fixed on the sheet by a fixing device.
The image forming apparatus employing such an electrophotographic technology, particularly, a full-color image forming apparatus or a high-definition image forming apparatus, often uses a two-component developer (hereinafter, merely referred to as “developer”) which is excellent in charge stability of a toner. The two-component developer contains a toner and a carrier. Friction is caused between the toner and the carrier while the two-component developer is stirred in the developing device. The friction allows the toner to be appropriately charged.
In the developing device, the developer is supplied on a surface of a developer carrying member (developing roller), and is caused to be uniform in thickness by a thickness control member. Then, the developer is carried to a developing region where the developer carrying member faces a toner image bearing member. In the developing region, a charged toner in the developer is transferred, by electrostatic force, to an electrostatic latent image formed on the toner image bearing member. This forms a toner image on the surface of the toner image bearing member in accordance with the electrostatic latent image.
These days, there is demand that an image forming apparatus be improved in image quality and energy saving property. In view of this, (i) a carrier having a small diameter in a range of 30 μm to 50 μm or (ii) a toner having a small diameter in a range of 5 μm to 7 μm and a low softening temperature are now used. However, a developer containing a carrier and a toner whose diameters are small is likely to congregate due to heat or stress.
In view of the problems, the following techniques have been proposed to reduce the stress on the developer.
For example, there has been proposed a developing device including a thickness control plate for causing the developer supplied on the surface of the developer carrying member to be uniform in thickness, which thickness control plate is constituted by a magnetic member and a non-magnetic member so as to (i) reduce the stress on the developer and (ii) stably ensure image quality (see Patent Literature 1). Further, there has been proposed such a technique that a magnetic member and a non-magnetic member are integrally formed with high accuracy by employing a laser welding technology (see Patent Literature 2).
Further, there has been proposed a developing device having another arrangement in which a rotatable developer control sleeve in which a magnet roller is provided is placed to face a developer carrying member (see Patent Literatures 3 and 4).